水跃和射流的掺气机理与湍流作用的比较性研究

Hydraulic jumps and plunging jets are common phenomena of flow discontinuity in open channel flows. Because of the associated free-surface breaking, air-water mixing, generation and dissipation of turbulent flow structures, as well as the limitation of two-phase flow measuring techniques, fundamental researches on self-aeration of such strong turbulent flows have been mostly limited to individual flow types under specified flow conditions. The lack of systematic understanding of fundamental problems such as air entrainment mechanisms and the interaction between turbulence and bubbly flow has placed some barriers in the way of accurate description and prediction of real problems with various engineering backgrounds. Herein, a comparative study of hydraulic jumps and two-dimensional planar plunging jets with similar inflow conditions is proposed. New experimental facilities are designed, and a variety of experimental approaches will be adopted to identify the spatial evolution of two-phase flow properties and the turbulence characteristics of the air-water flow. This will enable a systematic comparison of the air entrainment processes, development of air-water flow fields and interplay between the bubbles and turbulence in hydraulic jumps and plunging jets with different upstream impinging slopes. With the support of the experimental data, the aim of the project is two-fold: firstly, to build an analytical model that depicts the development of air-water flow field under different inflow conditions and a general air entrainment theory valid for both hydraulic jumps and plunging jets; secondly, to propose a systematic methodology for quantitative assessment of the level of coupling between turbulence and air-water two-phase flow in hydraulic engineering aeration problems.

水跃和射流是明渠流动中常见的流动不连续现象。由于伴随着自由面破碎、气液混合、湍流结构的生成与耗散等复杂物理过程，且受限于两相流测量技术，针对以水跃和射流为代表的强湍流自掺气流动现象的基础研究往往局限于单一流态的特定流动条件下，对掺气机理、湍流-两相流耦合作用机制等问题的认识缺乏系统性，难以对不同应用背景的实际问题进行准确描述和有效预测。本项目拟通过采用专门设计的水利实验平台，对水跃和二维平面射流进行相似流动条件下的对比实验研究。采用多种实验手段，测量两相流参数的空间演化和气液混合区域的湍动特征，系统比较多入射角度下水跃和射流的掺气机理、两相流场形态以及两相流场和湍流场的相互作用规律。项目的最终目标是在实验数据支持下，首先提炼适用于多种來流条件的两相流场数学模型，尝试构建水跃射流掺气的统一理论；其次，整合湍流-两相流相互作用的量化方法，以期形成一套适用于水利工程两相流研究的湍流掺气耦合关系评价体系。

水利工程泄洪消能普遍依靠水跃、射流等伴有剧烈湍流生成和耗散的流态突变过程，其自由面破碎、液滴飞溅、空气卷吸、气泡扩散等复杂现象的伴生和相互作用为相应水气二相流流态观测和水动力学模型构建设置了巨大障碍。本项目在自主研发水气二相流测量技术的基础上，对水跃、射流等典型掺气流态的二相流结构的形成和发展规律进行了研究。在不同来流入射角度和掺气条件下，对掺气机理及掺气浓度、气泡的密度、级配分布和聚类方式、流速和能量的衰减、湍流特征尺度的演变等性质进行了全面分析，并着重从以下方面开展了深入研究：1）介入和非介入式水气二相流观测技术；2）预掺气对局部气泡卷吸或逃逸以及掺气水体二相流结构发展的影响；3）原型及大尺度模型观测和二相流测量的比尺效应。此外，拓展研究了仿生植被边界对明渠泄洪中上述二相流特征、自掺气行为和消能效果的改变。截至目前，发表SCI收录的学术论文16篇，取得的成果为一系列后续研究打下了良好基础。